Is it legitimate to add more than two corresponding authors to a single article?

In certain cases, particularly for collaborative or interdisciplinary research projects involving multiple principal investigators or co-leads, it may be appropriate to designate more than one corresponding author.

I would say max 2, and in most cases 1 should be enough, it's confusing to have more. If there are many expertises or institutes involved, a corresponding author could forward any questions to their relevant co-authors. 

Yes, as in some instances, especially those involving multi-disciplinary research, more than two corresponding authors for an research manuscript may be unavoidable. 

Sometimes it is appropriate giving the fact that circumstances may prevail against one corresponding author or such a one may unduly modify the equally contributed manuscript towards insignificant idea/areas 

Two corresponding authors should be the upper limit. For a research, there is always one team leader. Two leaders for one work is exceptional. How can three or more leaders to guide or correspond to the research？

There is no internationally established limit on the maximum number of authors who can designated as corresponding authors in a scientific article. However, it is frequently advised to have a single recognised author should manage correspondence. Nevertheless, several journals may permit the inclusion of numerous corresponding authors in specific situations.

The selection of the corresponding author should be based on their substantial contribution and in-depth understanding of the research, their proficiency in effective communication, and their dedication to maintaining ethical standards throughout the publication process.

No, I don't see any substantive reason. The corresponding author is to correspond with the Editor. One is what makes sense. For technical and scientific issues related to the manuscript, he discusses them with fellow co-authors...

 

No issue in sharing correspondence, when the journal accepts it then it is not a problem, even some journals accept more than two as well, especially in multidisciplinary research.  

As per the contribution and requirement of the system there is no harm to have two corresponding authors.